This invention relates to aircraft distance measuring equipment (DME), and more particularly to improvements in digital DME systems.
Conventionally, DME operation involves a transmission of a radio signal of specified character (typically, pulse pairs at approximately 30Hz) from an aircraft to a ground station. After specified delays and under known constraints at the ground station, a replay signal is radioed back to the aircraft. Once the return signal is identified, and known time delays are accounted for, the duration between transmission and reception is decoded to yield distance from the ground station to the aircraft. Many aircraft utilize the same ground stations at the same time, however, so the aircraft receiver must contain logic which speedily and reliably discriminates the response to its own transmitted signal from responses which are being sent out to the other aircraft.
It is a primary object of the present invention to provide DME timing methods and apparatus whereby the discrimination process is facilitated.
Since operational conditions at the ground station may vary considerably, it is desirable that the DME transceiver adapt its own transmission to the rate of receipt of the ground station. For example, the response efficiency of the ground station is affected directly by the number of aircraft with which it is presently communicating. Hence, some signals arriving during certain time periods simply will not be favored with a response. Further, DME ground stations have a built in preference for higher powered signals, thereby substantially impairing the response efficiency to the lower powered signals.
It is accordingly an object of the present invention to condition transmission of subsequent signals partially upon prior receipt of decoded signals from the ground station.
Another conventional aspect of DME ground stations is the generation of TACAN, or modulation of transmitted pulses onto a relatively low frequency envelope. In order to accentuate response efficiency, it is therefore desirable to avoid transmission during the relative null due to path loss of the ground station created when the envelope is at its minimum amplitudes.
It is accordingly a further object of the present invention that transmission be synchronized selectively with the receipt of signals from the ground station, thereby insuring subsequent transmission to the ground station during desirable time periods.
Finally, since many aircraft concurrently utilize the ground station, provision must be made so that the various aircraft DME's do not become synchronous with one another, thereby obviating the possibility of meaningful response from the ground station. To this end, a certain amount of jitter is provided, whereby successive aircraft transmissionreception cycles are slightly variable in time relative to each other. One way in which this was done in the prior art was to utilize the noise generated by a high voltage neon bulb, or the like.
It is an object of the present invention to provide timing apparatus and methods whereby the requisite jitter is provided.